Brines are widely used in well treatment operations. For instance, brines are used in well completion as a kill or spike fluid, completion fluid, packer fluid, workover fluid, etc.
Typically, brines are clear and are characterized by a density ranging from about 8.5 pounds per gallon (ppg) to about 20 ppg. Low density brines are usually single salt fluids e.g., NaCl, NaBr, CaCl2 or CaBr2 salt in water, and the density may be as high as 12.7 ppg (NaBr) and 15.3 ppg (CaBr2). Mid-range density fluids, ranging from 11.6 ppg to 15.1 ppg, are typically two salt solutions, such as CaCl2/CaBr2. High density brines, having a density from 14.5 to 19.2 ppg, typically contain zinc or are composed of three salts. Solid-free brines, commonly used in drilling and completion and workover fluids, are typically high density brines. Table I illustrates exemplary brines with their respective density range:
TABLE IAqueous Brine CompositionBrine Density Range, ppgNaBr 9.0-12.7CaCl2 9.0-11.8CaBr210.0-15.3KCl8.5-9.7NaCl 8.5-10.0NaCl/NaBr 9.0-12.7CaCl2/CaBr211.7-15.1ZnBr214.5-20.5CaBr2/ZnBr214.5-20.5ZnBr2/CaBr2/CaCl215.0-19.2
Operators typically choose the brine for a treatment operation based on the true vertical depth (TVD), bottomhole pressure (BHP) and bottomhole temperature (BHT) of the well being treated. Further, since the brine is subjected to heating and cooling during the treatment operation, the temperature profile along the entire path to which the brine is exposed during the treatment operation is also considered.
Typically, the lowest temperature in the path of the brine is not lower than the true crystallization temperature (TCT) of the brine. The TCT (or saturation point) of a brine is the temperature at which a solid phase begins to form, resulting in a mixture of solid particles and solution. These solids may be salt crystals or water crystals, i.e., ice. The TCT is based on the salts dissolved in the brine and is dependent on the weight percent of salt in the brine. TCT typically increases with increasing brine density after the eutectic point, as illustrated in FIG. 1.
The TCT for a calcium bromide brine is provided in Table II.
TABLE IICaBr2,DensityDensityTCT,TCT,% wt.lb/galg/cc° F.° C.08.351.00320109.101.0925−42010.011.2014−103011.101.33−4−203511.681.40−15−264012.361.48−35−374513.111.57−62−525013.941.67−31−355114.111.69−15−265214.281.711−175314.441.7314−10
A typical crystallization temperature curve for a brine is the phase diagram illustrated in FIG. 2 for calcium chloride where water has been added to the brine to lower the TCT. The left side of the curve slopes downward with increasing concentration of calcium chloride and represents the solution and ice crystals. It represents the freezing point of the brine, where ice crystals begin to form. The right side of the curve represents the solution and salt crystals. It represents the phase boundary of the brine, below which the salt crystals begin to form. The eutectic point, minimum point where the two curves intersect, indicates the chemical composition and temperature at the lowest melting point of the components of the brine. It is the point at which the minimum crystallization temperature can be realized, i.e., where the solution is in equilibrium with two solid phases: ice and calcium chloride.
FIG. 3 compares the crystallization curve (water based) for a calcium bromide brine with the curve for the calcium chloride brine. FIG. 4 compares the crystallization curve for a potassium chloride brine and a sodium chloride brine with calcium chloride. The TCT for each is measured under atmospheric pressure. In each of these figures, solid salts form once the temperature of the brine is cooled below its TCT. Typically, the TCT is the determining factor when selecting a clear brine fluid for a completion/workover application.
Lowering of the TCT past its eutectic point results in the salting out of crystals, typically small angular particles, that seed crystal formation. Once formed, such masses of salt crystals are difficult to remove and can block any system using the brine. Thus, the formation of solids puts increased demands on pumping equipment in light of the increased resistance to fluid flow. In addition, the loss of soluble salts by settling or filtration drastically reduces the density of the completion fluid. Loss of density of the clear brine can result in an underbalanced situation.
Completion fluids are typically high density brines. Generally, a zinc bromide (ZnBr2) brine is used when a brine is required to have a density of about 14.5 ppg or above. Zinc bromide, blends of zinc bromide and calcium bromide and blends of zinc bromide, calcium bromide and calcium chloride have been preferred completion fluids in the Gulf of Mexico in wells that require density ranges 14.5-19.2 ppg.
At the conclusion of any completion or workover project, a substantial volume of contaminated or unneeded completion/workover fluid typically remains. Such fluids may be contaminated with water, drilling mud, formation materials, rust, scale, pipe dope, viscosifiers, fluid-loss-control pills, etc. Before bringing the well into production, removal of the brine is typically required. If it is deemed that the fluids have future use potential, they may be reclaimed and re-used. Zinc based brines have been proven to be difficult to reclaim and typically have little practical or economic value. Environmental regulations for conducting disposal activities and reporting and reacting to spills are heightened when a zinc containing brine is used.
Alternatives for high density brines, especially brines not containing zinc, are therefore desired.
It should be understood that the above-described discussion is provided for illustrative purposes only and is not intended to limit the scope or subject matter of the appended claims or those of any related patent application or patent. Thus, none of the appended claims or claims of any related application or patent should be limited by the above discussion or construed to address, include or exclude each or any of the above-cited features or disadvantages merely because of the mention thereof herein.